Bridesmatron???

So I asked my sister in law if she would be a bridesmaid and she got offended because as a married lady I should have asked her to be a bridesmatron and not a bridesmaid. After several “I didn’t know that was a thing” comment she flat out said she cannot be in my wedding if she isn’t titled a bridesmatron as opposed to a bridesmaid. All the other girls in my wedding party who have accept the invitation are all married and didn’t seem to care that I asked them to be a bridesmaid. This has caused quite a rift between me and sister in law. Is bridesmatron really a thing or is my sister in law just being a diva and what should I do in this situation?